As a laser source having broad application prospects, the fiber laser has advantages of a tunable bandwidth, a higher signal-to-noise ratio, and a narrower output laser linewidth, and can be widely used in fields such as fiber sensing, optical fiber communication and optical processing. The fiber laser comprises three parts of a pumping source, a resonator cavity and a gain medium. Longer the length of the cavity of the fiber laser, the nonlinear effect of the fiber laser is much substantial, thus it is necessary to shorten the length of the fiber. Meanwhile, the short cavity is an important condition to achieve single longitudinal mode operation for fiber laser. Short cavity fiber laser has a simple structure and is easy to be implemented. The short cavity fiber laser is typically consists of a pair of fiber gratings, and a gain medium connected therebetween, and this structure is called Distribute Bragg Reflection (DBR) type fiber laser. The short cavity fiber laser is usually used to generate narrow linewidth laser output. A U.S company NP Photonics utilized the 2 cm long erbium-doped phosphate glass fiber (DBR) laser to obtain the laser output with the power of 100 mW and the linewidth of 2 kHz. In 1992, Ball and others achieved a 1548 nm signal frequency output with the linewidth of 47 kHz which is consistent with Bragg wavelength using a 980 nm LD pump source by adding two Bragg gratings in the two ends of the 50 cm long Er3+ doped fiber for the first time, which the two bragg gratings are 1.25 cm long with the same bragg wavelength, and reflectance of 72% and 80% respectively. In 2007, A-FR company developed a type of fiber laser with the cavity length less than 5 cm, linewidth less than 3 kHz and output power up to 150 mW.
The short cavity fiber laser has several advantages such as a few numbers of longitudinal mode output, and stable output with no mode-hopping phenomena, and it is often used in field of fiber sensing. Therefore, there are important theoretical significance and application value to design a sensing system based on longitudinal mode output by the short cavity fiber laser.
The measurement of the temperature and the study of temperature sensor are important aspects in the field of sensing system. There are many methods for measuring the temperature, which can be divided into two categories, the contact temperature measuring method and the non-contact temperature measuring method according to whether the measurement body is in contact with the medium to be measured. The contact temperature measuring method is characterized in that the temperature sensing element is directly in contact and sufficiently exchanges heat with the object to be measured, and finally the thermal equilibrium is reached. The value of a certain physical parameter of the temperature sensing element represents the value of temperature of the object being measured at this time. The advantage of this temperature measurement method is intuitive and reliable, but the defect is the temperature sensing element will affect the distribution of the temperature field to be measured, and poor contact will bring measurement error. Furthermore, over high temperature and corrosive medium will bring adverse effects to the performance and life of the temperature sensing element. Non-contact temperature measuring method is characterized in that the thermal components do not contact with the object to be measured, but exchange the heat through the heat radiation, so it can avoid the disadvantage of the contact temperature measuring method and has a high limit for temperature measurement. In addition, the thermal inertia of the non-contact temperature measurement method is small, which is up to 1/1000 second, so it is convenient for measuring the temperature of the moving object and the rapidly varying temperature. However, the measurement error of this method is large due to the influences of the object emissivity, the distance between the object to be measured to the instrument, and other media such as dust, vapor, etc. According to the above two methods of temperature measurement, the thermometer also be divided into contact and non-contact thermometer.
As the longitudinal mode output of short cavity fiber laser can be used as a sensing system, there is a need for a method and system for accurately measuring temperature of an object utilizing the features of the short cavity fiber laser.